1. Field of the Invention
The present invention relates to an information reproducing apparatus, to which a maximum likelihood decoding method such as Viterbi coding is employed. More particularly, the present invention relates to an information reproducing apparatus, which controls a clock signal supplied to a maximum likelihood decoder so that the clock signal is not supplied thereto except when it is required, and thereby, can reduce a power consumption by the maximum likelihood decoder.
2. Description of the Related Art
In recent years, various recording and reproducing methods have been studied in order to realize high-density recording with respect to a recording medium. In particular, in the field of optical disk an MSR (Magnetically Induced Super Resolution) method has been developed, and it has been expected to realize excellent high-density recording. According to the above MSR method, it is possible to reproduce information having a bit smaller and a laser spot diameter determined by a wavelength and a numerical aperture (NA) of lens.
Moreover, as a transmission method in the case of recording and reproducing a data, a partial response method positively using intersymbol interference has been developed. Thus, it is expected to realize a practical use of information reproducing apparatus employing the partial response method and the Viterbi decoding method, which is one kind of the maximum likelihood decoding methods.
FIG. 1 is a block diagram showing the entire configuration of a conventional recording and reproducing apparatus with respect to a magnet-optical disk having a reproducing system performing Viterbi decoding.
In recording, a controller 2 receives a user data to be recorded pursuant to the instruction from the host computer 1, and then, encodes the received data based on the user data used as an information word, and therefor, generates an RLL (1, 7) code used as a code word.
In this case, the above RLL means a Run Length Limited method of limiting the number of level “0” between levels “1” and “1” in block coding. This is a coding method employed in order to secure an improvement of recording density and a stability of reproducing operation.
Then, the code word is supplied as a recording data to a laser power controller section (LPC)4. The controller 2 carries out operations such as decoding described later, control of modes such as recording, reproduction, and deletion, and exchange with the host computer 1, in addition to the processing as described above.
The laser power controller section 4 controls a laser power of optical pickup 7 in accordance with the supplied recording data so that a string of bit having a magnetic polarity is formed on a magnet optical disk 6, and thereby, performs recording. The above recording is a light intensity modulation, and a magnetic head 5 gives a bias magnetic field to the magnet optical disk 6. In fact, mark edge recording is performed according to a pre-code output generated based on the recording data, as described later.
To give an example of the method of recording a pre-code generated based on the recording data, as shown in FIG. 2, there is a mark position recording method such that a bit is formed with respect to “1” during pre-code output, and no bit is formed with respect to “0”. On the contrary, there is a mark edge recording method such that an inversion of polarity in boundary between bits during pre-code output corresponds to “1” expressed by the edge of bit. The following description is the case where the pre-code output is recorded by the above mark edge recording method.
Next the following is a description on the configuration and operation of a reproducing system. The optical pickup 7 irradiates a laser beam to the magneto-optical disk 6, and then, receives a reflecting light generated by the irradiation so as to generate a read signal. The read signal is composed of four signals, that is, a sum signal R+, a difference signal R−, a focus error signal (not shown) and a tracking error signal (not shown).
The sum signal R+ is adjusted in its gain by an amplifier 8, and thereafter, is supplied to a changeover switch 10. The difference signal R− is adjusted in its gain by the amplifier 9, and thereafter, is supplied to the changeover switch 10. Moreover, the focus error signal is supplied to means (not shown) for correcting a focus error. The tracking error signal is supplied to a servo system (not shown), and then, is used in the operation of the servo system.
The changeover switch 10 is controlled in the following manner. More specifically, the sum signal R+ is supplied to a filter section 11 for the duration that a read signal reproduced from a magneto-optical disk formed by embossing is supplied to the changeover switch 10. On the other hand, the difference signal R− is supplied to the filter section 11 for the duration that a read signal reproduced from a magneto-optically recorded portion of the magneto-optical disk is supplied to the changeover switch 10. A changeover signal S uses a signal extracted from a sector mark having a predetermined pattern.
The above filter section 11 is composed of a low-pass filter performing a noise cut, and a waveform e performing a waveform equalization. Moreover, it is preferable to employ a characteristic adaptable to Viterbi decoding method carried out by a Viterbi decoder 13 as a waveform equalization characteristic is used in waveform equalization processing.
An output of the filter section 11 is supplied to an A/D converter 12, and then, the A/D converter 12 samples a read signal value z[k] according to a read clock DCK.
The Viterbi decoder 13 generates a decode data based on the read signal value z[k] according to the Viterbi decoding method. The decode data is the maximum likelihood decode sequence with respect to the recording data recorded in the manner as described above. Thus, in the case were there is no decode error, the decode data coincides with the recording data.
The decode data is supplied to the controller 2. The controller 2 carries out decoding corresponding to coding such as the above-described channel coding with respect to the decode data, and then, makes a data processing instruction to generate a user data.
Moreover, the output of the filter section 11 is supplied to a PLL section 14. The PLL section 14 generates a read clock DCK based on the signal thus supplied. The read clock DCK is supplied to each of the controller 2, the A/D converter 12 and the Viterbi decoder 13, and processing is performed at a timing of the read clock DCK.
Further, the read clock DCK is supplied to a timing generator (not shown). The timing generator generates a signal for controlling a device operation timing of the changeover of recording/reproducing operation.
In the above reproducing operation, in order to obtain a more correct reproduction data based on the read signal read from the magneto-optical disk 6, each operation of constituent elements of reproducing system is justified in accordance with the quality of read signal. This operation is called as calibration.
The above calibration is performed in order to justify parameters of the reproducing system. More specifically, the calibration is carried out so as to be adapted to a possibility such that the quality of read signal changes depending upon a characteristic of recording medium by machining accuracy, a variation of recording laser beam power, recording/reproducing conditions by ambient temperature.
The content of calibration is, for example, adjustment relative to the following matters; more specifically, a read laser beam power of optical pickup, a gain of amplifiers 8 and 9, a waveform equalization characteristic of the filter section 11, and an amplitude reference value used in the operation of Viterbi decoder 13, etc. The calibration is carried out by the configuration (not shown in FIG. 1) after a power is turned on or when replacing a recording medium with another recording medium.
By the way, the Viterbi decoder 13 used in the above signal processing system is composed of a branch metric unit (BMU circuit), an adder-comparator-selector circuit (ACS circuit) and a status memory unit (SMU circuit), which will be described later. More specifically, the BMU circuit calculates a branch metric between the adjacent points of time. The ASC circuit calculates a plurality of path metrics, which is the sum of the branch metric of state transition between plural points of time, and selects the maximum likelihood state transition from there. The SMU circuit generates a status data sequence.
These circuits are relatively complicated, and use many circuit elements. For this reason, it has been known that a power consumption of the Viterbi decoder 13 becomes larger than other circuit systems. Therefore, if the power consumption of the Viterbi decoder 13 is reduced as much as possible, and thereby, it is possible to reduce the entire power consumption of the magneto-optical disk drive.